Why can't I change my profile image?

In Math.SE I can visit the 'Edit profile and settings' page and edit my display name, location, etc. But when I tried to upload a new profile image from my computer, it always fail.

Also, I can only view users' default profile images. I suppose that other users' images, which is blank from my point of view, should be images they uploaded themselves that my browser failed to load.

Is that only users with enough reputation can change their profile images, or there's some problem with the system? Thank you for helping me.

• There are some requirements on the file format and dimensions if the image that can be somewhat restrictive. What image are you trying to use? – Alexander Gruber May 19 at 4:32
• @AlexanderGruber A JPG format picture featuring fractals, its about 60KB. I downloaded it from internet. It's not a precise square. Should I modify the picture somehow? – Edit profile and settings May 19 at 4:52
• Side comment, unrelated to your current problem: depending on where you downloaded it (more specifically, depending on its license), you are not necessarily allowed to reuse it. – Jean-Claude Arbaut May 19 at 7:39
• @Jean-ClaudeArbaut Would you mind showing where to find a "legal" image? – Edit profile and settings May 20 at 12:00
• Anywhere, as long as there is a license that specifically grants the right to reuse the image. There are specialized sites, try with your favourite search engine. An image without an accompanying license is not free to reuse. – Jean-Claude Arbaut May 20 at 12:09

$$1$$. The reason why I can't load other one's images before is the firewall. Now a VPN is arranged so the problem is gone.
$$2$$. The photo shot by yourself can be uploaded as the profile image.